In intaglio printing presses, it is commonly known to use a wiping cylinder contacting the plate cylinder carrying the intaglio printing plate or plates as a wiping device for wiping and cleaning the surface of the intaglio printing plate or plates. The purpose of such a wiping cylinder is to simultaneously press the ink deposited onto the printing plates into the engravings and clean the excess ink from the plenum of the printing plates, i.e. the unengraved area of the printing plates outside the engravings.
In order to achieve good printing quality, the wiping cylinder is commonly designed in such a way that its outer surface contacting the printing plates is both physically and chemically resistant, i.e. is adapted to sustain the high contact pressure and friction with the printing plates and can withstand the physical and chemical contact with the ink components and pigments, as well as with the cleaning solutions which are used to clean the surface of the wiping cylinder.
It has already been proposed to provide such a wiping cylinder with an outer layer of resilient synthetic composition, namely a heat-hardenable plastic composition such as PVC. U.S. Pat. Nos. 3,785,286, 3,900,595 and 4,054,685 for instance disclose methods for making such wiping cylinders as well as apparatuses for implementing the said methods. These publications are incorporated by reference in the present application, especially in respect to the material used for forming such cylinders and to the machines and methods used for building such wiping cylinders. Referring for instance to the coating apparatus described in U.S. Pat. No. 4,054,685, means are provided for mounting a cylinder to be coated for horizontal rotation about its axis of rotation. Coating is performed by rotating the cylinder past a coating unit consisting of a straight-edged scraper blade mechanism disposed at one side of the cylinder and which extends parallel to the cylinder axis, this blade mechanism being adapted to be moved towards and away from the cylinder. The blade mechanism consists of two blades mechanically coupled to each other, namely a lower blade and an upper blade which are jointly designed to ensure a proper supply of heat-hardenable plastic material to the surface of the cylinder to be coated and allow adjustment of the thickness of the material to be deposited. The blade mechanism is adapted to be moved towards and away from the cylinder while maintaining the straight edge of the lower blade (i.e. the edge which extends along the length of the cylinder) parallel to the axis of rotation of the cylinder. The plastic material is supplied to the blade mechanism on top of the upper blade which is disposed, during coating of the cylinder, in an inclined relationship with respect to the cylinder so as to form a reservoir between the upper side of the upper blade and the periphery of the cylinder to be coated. Means are provided for restraining flow of the plastic material sideways from the reservoir. The blade mechanism can be translated towards and away from the cylinder in order to maintain a desired uniform spacing (a couple of millimeters or less) between the straight edge of the lower blade and the periphery of the cylinder along the full length of the cylinder. The cylinder is rotated in a direction to cause its periphery to move downwardly past the blade mechanism to thereby apply to the periphery of the cylinder a thin uniform layer of plastic composition having a thickness determined by the spacing between the straight edge of the lower blade and the periphery of the cylinder. This layer of plastic material is heat-cured by applying radiant heat to the cylinder throughout its length as the cylinder is rotated so as to cause hardening of the deposited layer of plastic material and produce a hardened layer of the desired hardness. Several layers with different hardnesses and thicknesses are preferably formed in this way onto the cylinder surface.
According to the solutions described in U.S. Pat. No. 4,054,685, the heating means for applying radiant heat to the cylinder are disposed in a movable hood part that can be displaced vertically on top of the cylinder. The hood part is designed in such a way that the cylinder is completely hidden below the hood part when the later is in place. Further not only is the cylinder completely hidden by the hood part, but also the coating unit. As a consequence, the coating process must be performed with the hood part moved up vertically so as to allow the user to supply the plastic composition on the coating unit and to visually monitor the coating process. It is only after the coating process has been performed that the hood part can be lowered onto the freshly coated cylinder to harden the deposited layer of plastic material. This prior art solution has a number of disadvantages including in particular a poor ability to aspirate the fumes generated during the coating process. Further, as heat has to be applied to the cylinder during the coating process, the operator is subjected to the heat generated by the heating means which has to be kept at a somewhat high level to compensate for the quick cooling effect caused by cool air flowing from all sides of the machine.
U.S. Pat. No. 5,180,612 discloses another type of apparatus for coating a wiping cylinder with a layer of plastic material which, in contrast to the previous apparatuses, makes use of a twin-roller coating unit for the application of the plastic material onto the surface of the cylinder. Rather than a vertically-moving hood part, there is provided a hood part that can be pivoted onto or away from the cylinder mounting location. A disadvantage of this solution however also resides in the fact that the operator cannot monitor the cylinder during the coating process, because the hood part completely hides the cylinder as well as the part of the coating unit which cooperates with the cylinder where the plastic composition is applied onto the cylinder. Further, the visibility of the cylinder is much more restricted with this solution due to the substantially greater size of the coating unit with its two application rollers. As a consequence, the operator must again open the hood part by an amount sufficient for him to be able to visually inspect the surface quality of the deposition, to the detriment of the efficiency of the aspiration of the fumes and of the heating.
Another disadvantage of the solution described in U.S. Pat. No. 5,180,612 resides in the structure of the heating means and aspiration system. Firstly, the heating means are disposed on a common reflector plate which constitutes an obstacle to the flow of air within the interior space of the hood part. Secondly, the part of the aspiration system disposed on the machine is entirely located in the hood part, an exhaust pipe being coupled directly to the hood part. This construction can cause problems because the exhaust pipe (as well as the other pipe elements connected thereto) will be subjected to the same rotational movement as that of the hood part.